The present exemplary embodiments relate to a method for forming ceramic thick film element arrays. It finds particular application in conjunction with the formation of ceramic elements such as piezoelectric thick film arrays, such as lead zirconate titanate (PZT) arrays, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiments are also amenable to other like applications such as production of other ceramic thick film arrays.
By way of background, thick (e.g., 10 to 100 μm thickness range) ceramic or piezoelectric material, such as PZT films, have many potential uses in micro-electromechanical (MEMS) devices, inkjet printers and ultrasonic transducers. However, producing films in this thickness range on commonly used substrates such as silicon, metal and plastic has been found to be very difficult. These substrates cannot withstand the temperatures used to sinter the ceramic thick films. Generally, it is beyond the ability of thin film methods such as sol-gel and sputtering to produce suitable devices. It is likewise beyond the ability of bulk ceramic processing to do so.
A number of processes for forming thin film materials or bulk materials are known. For example, U.S. Pat. No. 6,071,795, entitled “Separation of Thin Films from Transparent Substrates by Selective Optical Processing,” discloses a method for separating a thin film of gallium nitride that is grown on a sapphire substrate. The thin film is bonded to an acceptor substrate, and the sapphire substrate is laser irradiated with a scanned beam at a wavelength at which the sapphire is transparent but the gallium nitride is strongly absorbing. After the laser irradiation, the sample is heated above the melting point of gallium, and the acceptor substrate and attached gallium nitride thin film are removed from the sapphire growth substrate.
Another method relating to the transfer of bulk and thin film materials is disclosed in U.S. Pat. No. 6,335,263, entitled “Method of Forming a Low Temperature Metal Bond for Use in the Transfer of Bulk and Thin Film Materials”. In this document, a method of forming a low temperature metal bond is disclosed as including a step of providing a donor substrate, having a thin film grown thereon. An acceptor substrate is then produced and a multilayer metal bond interface for positioning between the thin film and the acceptor substrate is then selected. A bonded layer is then formed between the thin film and the acceptor substrate using the multilayer metal bond interface. The donor substrate is then severed from the thin film to isolate the thin film for subsequent processing.
Both of these methods contemplate the use of sapphire. As those of skill in the art will appreciate, sapphire is expensive and may, thus, render implementation on a large scale impractical. Both of these methods also contemplate the use of irradiation, e.g. laser lift-off, to release elements from a substrate.
Moreover, conventional ceramic thick films, such as screen-printed PZT films, need to be sintered at more than 1100° C. Thus, only a few substrates—such as aluminum oxide or zirconium oxide—can be used. Therefore, even if one were to attempt to adapt and use the above referenced thin film applications for thick film applications, there are several apparent drawbacks. First, such a method of production would require a large, up-front investment to buy the expensive sapphire substrates. The expected return (e.g. profit) to be enjoyed by the resultant ceramic material made from this process would typically not justify the cost of the sapphire substrates. Secondly, sintering the ceramic elements at 1250° C. or higher in a lead rich environment would surely result in some diffusion or inter-reaction, i.e. undesired bonding, between the ceramic, or PZT, films and the substrate. This will make the contemplated laser liftoff more difficult and may increase the process cost.
Accordingly, an improved and more efficient process is desired to transfer ceramic elements, such as thick film PZT elements, from a substrate upon which they are formed (but which does not comprise a sintering substrate), without using an optical or radiation technique.